(1) Field of the Invention
The present invention relates to solid-state imaging devices, such as Charge-Coupled Device (CCD) image sensors, and a method of driving the devices, and more particularly to a technology of reducing noise such as smears in pixel thinning driving.
(2) Description of the Related Art
Conventionally, solid-state imaging devices which convert incident light into electronic signals to be outputted as video signals have been known. Cameras, such as digital still cameras, which display as still images the video signals obtained by such solid-state imaging devices have also been known. In recent years, further improved image quality and functions have been demanded in the cameras using the solid-state imaging devices. To satisfy the demands, technologies of high pixel density have been developed.
Regarding such solid-state imaging devices, Japanese Patent Application Publication Nos. 9-298755 and 11-234688 (hereinafter, referred to as Patent References 1 and 2, respectively), for example, disclose a driving method, which makes it possible to reduce the number of pixels in output video signals, by selecting the number of pixels from which signal charges (hereinafter, referred to simply as “charges”) are read out in order to reduce the pixels (hereinafter, the selecting is expressed as “thinning”). As a result, a speed of the outputting is increased especially in a preview mode and the like.
However, in such thinning driving, noise components, such as smears and dark currents, are problematic. This problem is explained in detail with reference to FIGS. 1A and 1B.
FIGS. 1A and 1B are schematic diagrams for explaining a problem in a conventional technology disclosed in Japanese Patent Application Publication No. 2000-299817 (hereinafter, referred to as Patent Reference 3). In FIGS. 1A and 1B, white circles represent noise components on a vertical transfer unit, and black circles represents signal components which are transferred once during a horizontal blanking interval.
As shown in FIG. 1A, in driving for reading charges from all pixels (hereinafter, referred to as “all-pixel readout driving”), which is used when still images are captured, for example (hereinafter, referred to as a “still image capture mode”), charges are simultaneously read out from all pixels (shown as black squares) to the vertical transfer unit. Then, the readout charges are independently transferred in a vertical direction without being mixed together in the vertical transfer unit. The transferred charges are then horizontally transferred in a horizontal transfer unit and read out via a charge detection unit.
In the all-pixel readout driving shown in FIG. 1A, during one horizontal blanking interval, each vertical transfer unit can transfer charges from only one pixel to the horizontal transfer unit. Furthermore, an amount of signals outputted at one time from an amplifier is signals of one pixel. An amount of noise outputted at one time from the amplifier is noise of one horizontal transfer stage (a noise amount of one packet). Here, it is assumed that an electronic shutter function is not used even in the all-pixel readout driving.
On the other hand, as shown in FIG. 1B, in driving for reading charges from only selected pixels (hereinafter, referred to as “thinning readout driving”), which is used when captured images are previewed, for example (hereinafter, referred to as a “preview mode”), if the thinning is performed every other pixel in a vertical direction, charges are read out from only pixels in odd-numbered rows (1, 3, 5, . . . in FIG. 1B) to the vertical transfer unit. Here, the vertical transfer unit also transfer empty packets. A pair of one signal packet and one empty packet is transferred in a vertical direction. Then, charges of the two packets are mixed and transferred by the horizontal transfer unit in a horizontal direction, and eventually detected by the charge detection unit.
In the above thinning readout driving shown in FIG. 1B, during a horizontal blanking interval, each vertical transfer unit can transfer charges from two pixels to the horizontal transfer unit. Furthermore, an amount of signals outputted at one time to the amplifier is signals of one pixel. An amount of noise outputted at one time to the amplifier is noise of two horizontal transfer stages (a noise amount of two packets).
As obvious from the above-explained processing, noise is increased by the thinning reading driving of FIG. 1B. The reason is explained in detail below. There are empty packets due to pixels from which charges are not read out, and these empty packets also accumulate noise on the vertical transfer unit. When charges in a pair of two vertical pixels (upper and lower packets) are mixed in the horizontal transfer unit, noise components of these two packets are included in a signal component regarding one pixel. As a result, a signal-to-noise ratio (dB) is increased to double (6 dB).
There have been various proposed technologies to solve the above problem. One of such technologies is disclosed in the Patent Reference 3. The following explains this solution.
FIG. 2 is a schematic diagram showing a solid-state imaging device according to the conventional technology. Here, it is assumed that this solid-state imaging device is a CCD imaging device which generally uses all-pixel readout driving, but can also use thinning readout driving by which thinning is performed every other pixel in a vertical direction.
As shown in FIG. 2, an imaging unit (imaging area) 11 is formed on a semiconductor substrate. The imaging unit 11 includes a plurality of light-receiving elements (pixels) 12 and a plurality of the vertical transfer unit 13. The light-receiving elements 12, such as photodiodes, are arranged by rows and columns in the imaging unit 11. Each of the light-receiving elements 12 converts incident light into signal charges whose amount corresponds to the light amount, and accumulates the charges. Each of the vertical transfer units 13 is arranged along a column of the light-receiving elements 12.
Each vertical transfer unit 13 transfers a set (packet sequence) of signal packets and empty packets. Each of the signal packets corresponds to each pixel. Each of the empty packets is transferred prior to each signal packet. Furthermore, three transfer electrodes (not shown) per one packet are formed on the vertical transfer unit 13 in a direction of transferring packets. Here, the “packet” refers to a unit of transfer on the vertical transfer unit 13. Furthermore, the vertical transfer unit 13 is driven to transfer the packets, by three vertical transfer pulses Vφ1, Vφ2, and Vφ3, for example.
More specifically, during a horizontal blanking interval, the vertical transfer units 13 transfer charges from one row (one line) to the horizontal transfer unit 14, according to the vertical transfer pulses Vφ1, Vφ2, and Vφ3 (hereinafter, this processing is referred to as “line shifting”). Here, a part of the transfer electrodes of the vertical transfer unit 13 serves also as a readout gate electrode, so that one of the vertical transfer pulses Vφ1, Vφ2, and Vφ3 has three different value levels; low, medium, and high. Among the levels, the high value pulse is used as a readout pulse.
Here, the three vertical transfer pulses Vφ1, Vφ2, and Vφ3 are provided to each packet in the thinning readout driving in the different manner as described for the all-pixel readout driving. That is, of course, the readout pulse is not provided to pixels from which charges are not to be read out. Moreover, in the all-pixel readout driving, the line shifting is executed only once per horizontal blanking interval, but in the thinning readout driving, by which thinning is performed every other pixel, the line shifting is executed twice per horizontal blanking interval.
Under the imaging unit 11 of FIG. 2, a horizontal transfer unit 14 is formed adjacent to ends of the vertical transfer units 13. The ends are at the side to which charges are transferred. In the thinning readout driving, the horizontal transfer unit 14 needs to transfer a signal packet and a noise packet as a pair, so that the number of packets arranged in the horizontal transfer unit 14 (a packet sequence) is as twice as the number of pixels in a horizontal direction (double density). The packet sequence is transferred by driven by horizontal transfer pulses Hφ1 and Hφ2 which are reversed phases from each other. Each of the horizontal transfer pulses Hφ1 and Hφ2 is set to have a frequency twice higher than usual, since the horizontal transfer unit 14 has double density.
At an end of the horizontal transfer unit 14, to which charges are transferred, there is a charge detection unit 15, such as a floating diffusion amplifier, which detects the charges transferred from the horizontal transfer unit 14 and converts the charges to a signal voltage to be outputted. The charge detection unit 15 includes a floating diffusion (FD) unit 17, a reset drain (RD) unit 18, and a reset gate (RG) unit 19. The FD unit 17 is formed adjacent to a final output gate 16 of the horizontal transfer unit 14. The RD unit 18 drains the charges. The reset gate 19 discharges the charges from the FD unit 17 to the RD unit 18.
In the charge detection unit 15, the RD unit 18 is applied with a predetermined drain voltage Vrd. The RD unit 19 is applied with the horizontal transfer pulses Hφ1 and Hφ2 and a reset gate pulse RGφ having the same cycles as the horizontal transfer pulses, for example. From the FD unit 17, a output signal voltage Vout converted from charges is obtained. Note that various timing signals including the vertical transfer pulses Vφ1, Vφ2, and Vφ3, the horizontal transfer pulses Hφ1 and Hφ2, and the reset gate pulse RGφ are generated by a timing generation circuit 20.
FIG. 3 is a timing chart showing a timing relationship among the horizontal transfer pulses Hφ1 and Hφ2, the reset gate pulse RGφ, and the output signal voltage Vout. In the waveform of the output signal voltage Vout, P phase is for presetting and D phase is for data outputting. Since the horizontal transfer unit 14 has double density in two-packet units, data of a horizontally prior packet in a packet pair is outputted in P phase, and data of a horizontally subsequent packet in the packet pair is outputted in D phase.
The following describes respective driving processing performed by the above-described CCD imaging device which usually uses all-pixel readout driving, with reference to FIGS. 4A, 4B, and 5A to 5C. FIGS. 4A and 4B are schematic diagrams showing processing performed by the conventional CCD imaging device in the all-pixel readout driving. FIGS. 5A to 5C are schematic diagrams processing performed by the conventional CCD imaging device in the thinning readout driving. In these figures, white circles represent noise components on the vertical transfer unit 13, and black circles represents signal components which are transferred once during a horizontal blanking interval.
Firstly, when the all-pixel readout driving is set by a driving switching signal, the timing generation circuit 20 outputs the vertical transfer pulses Vφ1 to Vφ3 whose timings correspond to the set driving. Thereby, as shown in FIG. 4A, charges are read out simultaneously from all pixels (shown as black squares) to the vertical transfer unit 13.
Subsequently, line shifting is performed in the vertical transfer unit 13. Here, prior to the line shifting, the horizontal transfer unit 14 beforehand shifts one bit (one packet) (hereinafter, referred to as “one-bit shifting”) in order to provide charges to a horizontally subsequent packet in a packet pair, since the horizontal transfer unit 13 transfer charges in two-packet units. After that, line shifting is executed once during a horizontal blanking interval. As a result, as shown as FIG. 4B, charges per one line (one row) are provided to the horizontal transfer unit 14.
As described above, the charges per one line provided from the vertical transfer unit 13 to the horizontal transfer unit 14 are transferred in a horizontal direction using the horizontal transfer pulses Hφ1 and Hφ2, and then sequentially inputted to the charge detection unit 15 in units of pixels. In the charge detection unit 15, the reset gate pulse RGφ is supplied to the reset gate unit 19 in the same cycles as the horizontal transfer pulses Hφ1 and Hφ2, thereby reset the reset gate unit 19 to discharge remaining charges from the FD unit 17 to the RD unit 18.
Thereby, from the FD unit 17, an output signal voltage Vout having the waveform shown in FIG. 3 is obtained. Note that, in the all-pixel readout driving, as obvious from the above-described processing, a prior packet in the packet pair is not provided with any data from the vertical transfer unit 13. Therefore, regarding the output signal voltage Vout, no data is outputted in P phase, and data of signal components is outputted in D phase. Note that the P phase becomes a reference phase for signal processing described later
On the other hand, when the thinning readout driving is set by a driving switching signal, the timing generation circuit 20 outputs the vertical transfer pulses Vφ1 to Vφ3 whose timings correspond to the set driving. Thereby, as shown in FIG. 5A, charges are read out from only pixels in even-numbered rows (2, 4, . . . in the figures) (shown as black squares) to the vertical transfer unit 13.
Subsequently, the first line shifting is executed in the vertical transfer unit 13 during a horizontal blanking interval. Thereby, charges of noise components per one line are provided from the vertical transfer unit 13 to a horizontally prior packet in the horizontal transfer unit 14. After that, one-bit shifting is executed in the horizontal transfer unit 14.
Next, the second line shifting is executed in the vertical transfer unit 13 during the same horizontal blanking interval. Thereby, charges of signal components per one line are provided from the vertical transfer unit 13 to a horizontally subsequent packet in the packet pair in the horizontal transfer unit 14. As a result, in the horizontal transfer unit 14, the charges of noise components are accumulated in the prior packet, and the charges of signal components are accumulated in the subsequent packet.
As described above, by executing line shifting twice within one horizontal blanking interval, charges of a noise component and charges of a signal component, which are provided from the vertical transfer unit 13 to the horizontal transfer unit 14, are transferred in a pair of packets in a horizontal direction, and sequentially inputted to the charge detection unit 15. The charge detection unit 15 is reset by the reset gate pulse RGφ having the same cycles of the horizontal transfer pulses Hφ1 and Hφ2. As a result, regarding the output signal voltage Vout having the waveform shown in FIG. 3, data of the noise component is outputted in P phase and data of the signal component is outputted in D phase.
By resetting the charge detection unit 15 in the same cycles as the transfer cycles of the horizontal transfer unit 14, resetting between P and D phases are also executed. As a result, data of a noise component per one packet is outputted in P phase, and data of signal and noise components per one packet is outputted in the D phase.
As described above, in the CCD imaging device basically using the all-pixel readout driving, when the thinning readout driving is set, charges are transferred in a pair of a signal packet and an empty packet in a horizontal direction, then the horizontally transferred charges are sequentially converted into electronic signals, and data of a noise component in the empty packet is outputted in P phase and data of a signal component in the signal packet is outputted in D phase. Thereby, it is possible to obtain a combination of (i) data of a pixel and (ii) data of noise in the same column of the pixel, in other words, data of noise in the horizontally same address as the pixel.
Therefore, in subsequent signal processing, by eliminating difference between the signal component in D phase and the noise component in P phase, it is possible to cancel noise component included in the signal component. Note that the noise component in the empty packet is smears, dark currents, and the like, which is noise in the same direction as the vertical transferring.
Moreover, Japanese Patent Application Publication No. 2005-328212 (hereinafter, referred to as Patent Reference 4) discloses a technology of removing smear components by detecting in a latter circuit the difference between (i) a signal component and a noise component read out from a readout pixel and (ii) a noise component included in an empty packet, and of forming a transfer blocking gate at a final part of a vertical transfer path. With the technology, it is possible to thin pixels not only in a vertical direction, but also in a horizontal direction.